changing LAB values

[jaramd]

In Photoshop, I create a color fill layer with the LAB color 80,120, 80 (RGB is 255,0,60). Then I go back with the eyedropper on that fill layer and the LAB color has changed to 55,81,45 while the RGB color values are the same at 255,0,60. Why does the LAB values change (although the actual color does not)?

[TheDigitalDog]

Need to see a screen capture but keep in mind, a Lab value is a fixed value, but the RGB values may not be (for example, if you converted from Adobe RGB (1998) to sRGB). 

Is it possible you've got a layer with some Blending Mode on?

What is the RGB Working Space? 

[TheDigitalDog]

Hold on, I just tried this in sRGB. Looks like a bug! Indeed, the info paleete showing Lab is wrong no matter the sampling size. I made an entire new document in sRGB with Lab 80,120, 80 and the foreground picker shows 255/0/60 yet the info  palette shows RGB is 255,0,60. That's messed up. 

I'll fill a bug report. 

[jaramd]

Wow!

Nice catch, thanks for checking it out... thanks...

[jaramd]

Thanks for the thoughtful response.. It was a layer with a blending mode.

Problem solved!! Thanks

[TheDigitalDog]

Solved how?

I had no layers, no modes and the numbers are wrong. 

[TheDigitalDog]

What OS? 

I'm on Big Sur and again, I see this very odd behvaior. 

[Chuck Uebele]

I believe L*A*B* values only go up to 100, not 255, like RGB, so I'm not sure how you got 120, but if you managed to input that, PS most likely adjusted the values.

[TheDigitalDog]

Lab value should go to -128/127. 

Again, all I did was make a new document in sRGB.

Foreground color set in Lab to 80/120/80. PS totally allows that. 

RGB values are what the OP reported. 

Fill with foreground color. 

RGB values are OK, Lab values are way off. 

Can someone else try this and let me know what they see? The lab values reported in the Info Palette should of course show 80/120/80. I get Lab values of 55/81/45. Boom, head exploding. 

[Chuck Uebele]

I was going to try, but my Adobe apps are all screwed up, due to some issue trying to verify my subscription status. Dead in the water right now.

[Chuck Uebele]

I'm not so sure that it's a bug as the fact that we're trying to force LAB color in the RGB space. Can't LAB color be imaginary colors: a yellow so bright that we and devices can't precieve or reproduce it? I think PS is dumbing down the LAB values to fit inside RGB. I seem to recall a trick that Dan Margulis did to bring back some color is blown out areas. He would use an intense LAB color and PS would try and convert it to something the device could render. His example was a gold steeple.

[D Fosse]

Not at my workstation to test right now - but are you all sure that Lab color is within sRGB gamut? It's at least right at the gamut boundary if it produces R 255 and G 0 right off.

[D Fosse]

Just checked that Lab color, and it's way way out of sRGB gamut. It's also slightly out of Adobe RGB gamut (which you can just see on a wide gamut monitor).

In other words, all bets are off here. Small inaccuracies can be expected at the gamut clipping point.

[TheDigitalDog]

I don't see gamut the issue; the reported numbers are the problem. It should be consistent in both locations and isn't. PS “knows” the color space when entering the numbers in the first place and presents them incorrectly.

[D Fosse]

Well, I still don't see anything unexpected here.

Lab 80-120-80 is far out of sRGB gamut, so naturally it gets clipped when pasted into an sRGB document. It gets clipped to 55-81-45. That's expected.

In sRGB, those numbers are 255-0-60. That's red at 255 and green at 0, so at the gamut clipping point in both the red and green channels.

[TheDigitalDog]

What is unexpected is the color picker is not color managed. The info palette obviously is. Maybe I have never noticed this in the past, but the picker just invents RGB numbers from Lab and it doesn't have to.

[TheDigitalDog]

OK, brain fart on my part largly due to missing the very important warnings in the picker for Web safe and gamut colors. But the behavior is still wonky IMHO. 

If you make a document in sRGB and select Lab values 80/120/80, make sure one radio button is on a Lab value, then click the warning, it updates to show the resulting vaules you'll get in RGB as 237/21/86. It isn't tied to the Proof Setup either, I messed with different color spaces there, I always get 237/21/86.

Fill the document and read the RGB values, clipped to sRGB and I get RGB 55-81-45. What's going on here? 

Make a document in ProPhoto RGB and use the same Lab values, we are shown prior to the 'gamut warning' the RGB values are 255/109/62. Click the warning icon, you end up with Lab values of 52/76/26. Don't let the triangle clip, keep lab values of 20/180/20, paste into the ProPhoto RGB doc, you get RGB values of 72/94/67.

Where are these values from Lab to RGB coming from? 

Here is what I think should happen in a perfect world:

No document open: RGB Working Space set in Color Prefs used to convert from Lab to RGB etc/

Document open and forefront in sRGB. Picker uses sRGB for conversions and OOG wanrings. That doesn't appear to be the case. 

Why are the clipping conversions with the warning shown below different than just pasting the values into a document and looking at the Info palette? 

[D Fosse]

I always assumed the warning triangle referred to working CMYK? Or am I wrong? I've always ignored those completely, never paid any attention to them. I have no idea what purpose they're supposed to serve.

So, still ignoring those two buttons, everything here is entirely consistent and the color picker is fully and correctly color managed. The numbers are as expected all the way:

Lab 80-120-80 > clipped to sRGB = 55-81-45.

In an Adobe RGB document, the Lab readout, clipped to gamut, is 63-91-57

All of these read as RGB 255-0-60 because the original Lab color is out of gamut in both sRGB and Adobe RGB.

[TheDigitalDog]

---

@D Fosse wrote:

I always assumed the warning triangle referred to working CMYK? Or am I wrong?

---

Changing the CMYK Working Space does change that clipping value so yes, that's correct.

OK, after coffee and looking this over AGAIN, maybe not an ideal behavior (no RGB gamut clipping warning) but at least some consistency with Lab to RGB reported in the picker and the RGB values after (with Lab values updated):

If the current doc is ProPhoto RGB the Lab values discussed 'map' to 255/109/62 in that picker. 

If the current doc is sRGB, the Lab values discussed 'map' to 255/0/60.

If no document is open, the Lab values discussed 'map' to 255/0/60 IF the RGB Working Space set  is sRGB. 

If no document is open, the Lab values discussed 'map' to 255/109/60 IF the RGB Working Space set  is ProPhoto. 

So picker looks at the current (open) doc and uses that color space for Lab to RGB even if Lab values can't be produced as RGB. Again, a warning would be useful about that conversion (NOT CMYK).

IF no document is open, picker uses RGB Working Space set in Color Settings for Lab to RGB, even if Lab values can't be produced as RGB. 

Again, a warning would be useful about that conversion (NOT CMYK).

Warning triangle when clicked 'converts' to CMYK setup in Color Settings. 

Lastly, I really wish the Lab scale in PS matched Lightroom which I think is more useful. It doesn't truncate Lab values to +/- about 128. So if you have ProPhoto RGB and ask for G255/B0/R0, that is an "illegal color" and LR reports the Lab values as -186 (not truncated) informing the Lab savvy user this isn't a color. 

[jaramd]

Yes, I confirmed that this is simply an out of gamut issue. When I created
a fill layer with an RGB file the LAB values were immediately changed.
But, when I created a file using LAB, the LAB values did not change.
Hence, clipping due to out of gamut range..